1. The effect of guanidine hydrochloride (GuHCl) and trimethyl-N-amine oxide (TMAO) upon the equilibrium dissociation of tetrameric to dimeric cyanmethemoglobin (CNmetHb) was studied by measurement of dynamic light scattering. This measurement yields the intensity average diffusion coefficient of the protein, which increases as the protein dissociates. The results showed that the increasing tendency of CNmetHb to dissociate with added GuHCl may be inhibited by addition of sufficient amounts of TMAO. The stabilizing effect of TMAO is attributed largely to volume exclusion (D. Wu). 2. The self-association of the bacterial septation protein FtsZ in the presence of GTP or a GTP analogue, GMPCPP, and 500 mM potassium was studied as a function of protein and Mg concentration using single-angle and multi-angle static light scattering. The results reveal that when the concentration of protein exceeds a Mg-dependent critical value, FtsZ undergoes a concerted formation, akin to a second-order phase transition to a narrow distribution of high molecular weight oligomers with a stoichiometry of ca 100 (GTP) or 160 (GMPCPP). Above the transition concentration, the amount, but not the size, of oligomer increases, while that of low molecular weight species remains roughly constant (Monterroso, Rivas). All of the data obtained from the several types of measurements may be accounted for semiquantitatively in the context of a model, according to which the stable oligomer is a cyclic species, the size of which reflects the tendency of the protein fibrils to flex or curve in solution in the presence of different concentrations of Mg and the particular guanine nucleotide present (B. Monterroso, G. Rivas, CIB).